Conventionally, a DC voltage signal is generated from a line voltage in order to supply a power to LEDs by means of a power factor correction circuit and a smoothing capacitor. This DC voltage signal is converted by pulse width modulation into a pulse-width-modulated DC voltage signal. A load, for example directly an LED module, or a further LED converter stage is operated by the pulse-width-modulated DC voltage signal. The output signal of the rectifier module in this case has a residual ripple at a frequency of twice the line voltage.
Even if, for example, a DC supply is present for emergency light operation, ripple may arise if the clocking of the PFC circuit is modulated deliberately in order to improve the EMI properties.
Customary frequencies for pulse width modulation are in the range of from 100 Hz to 500 Hz. In particular when the frequency used for the pulse width modulation is close to the frequency of the residual ripple of the DC voltage signal or an integral multiple of this frequency, low-frequency beats occur as a result of demodulation. In the case of an LED module, these beats result in a clearly visible fluctuation in brightness at the frequency of the beat, i.e. at the differential frequency between twice the line frequency or the integral multiple thereof and the frequency of the pulse width modulation.
For example, US 2011/0043133 A1 discloses an operating circuit for operating an LED module which generates a DC voltage signal with a residual ripple by means of power factor correction from an AC voltage. This signal is processed by pulse width modulation and is used for operating the LEDs. In this case, the frequency of the pulse width modulation is set independently of the line frequency. The circuit arrangement disclosed therein is disadvantageous since it can result in beats, as described above.
The present invention is based on the object of providing a method and a circuit arrangement which, independently of the line frequency used, enable operation of light-emitting means without the occurrence of undesirable fluctuations in brightness which are visible to the human eye.